The teacher told the student to abridge the essay he had written.

amplify
shorten
increase
modify

The correct answer is B. shorten.

To abridge something means to make it shorter. In the context of the question, the teacher is telling the student to make his essay shorter. This could be done by removing unnecessary information, combining sentences, or using shorter words and phrases.

The other options are incorrect. To amplify something means to make it louder or stronger. To increase something means to make it bigger or more numerous. To modify something means to change it in some way.